1. Field of the Invention
The present invention relates to a power steering system.
2. Description of the Related Art
A power steering system has been widely used in an automobile or an industrial vehicle as a device for assisting the steering operation of a steering wheel. Especially, for an industrial vehicle such as a forklift, etc., the steering operation of a steering wheel is heavy if the vehicle does not comprise a power steering system. Therefore, such an industrial vehicle is equipped with a power steering system in most cases.
FIG. 1 schematically shows an existing power steering system. In this system, when an operator or a driver steers a steering wheel 101, a sensor (such as a rotational angular sensor or a torque sensor) 103, which is attached to a steering shaft 102, detects this steering operation. A controller 104 controls a motor 106 for driving an oil pump 105 based on the output of the sensor 103. In this way, an oil pressure (output) corresponding to the steering operation of the steering wheel 101 is generated. Then, the generated oil pressure is transmitted from a PS valve 107 to a PS cylinder 108. This oil pressure generates a thrust for changing the angle of wheels 110, so that the angle of the wheels 110 is controlled.
In the system having the above described configuration, when the steering wheel 101 is turned fast, a large output is required to quickly change the angle of the wheels 110. Therefore, the controller 104 rotates the motor 106 at a high speed. In the meantime, when the steering wheel 101 is turned slowly, a small output is sufficient. Accordingly, the controller 104 rotates the motor 106 at a low speed.
The controller 104 controls a revolution speed of the motor 106 with a PWM (Pulse-Width Modulation) method. That is to say, when rotating the motor 106 at a low speed, the controller 104 provides the motor 106 with a pulse signal having a small duty (duty cycle or duty ratio) as a control signal. In the meantime, when rotating the motor 106 at a high speed, the controller 104 provides the motor 106 with a pulse signal having a large duty. Note that xe2x80x9cdutyxe2x80x9d is a time ratio of an operation period in a cycle to a entire period of the cycle.
FIG. 2A explains a motor controlling method for use in the existing system shown in FIG. 1. Here, the revolution speed of the motor 106 is determined by the duty of the control signal generated by the controller 104.
With this system, the revolution speed of the motor 106 is controlled to be low so as to consume less power while the steering wheel 101 is not being turned. In the example shown in FIG. 2A, the controller 104 controls the motor 106 at a duty of 30 percent, during the steering wheel 101 is not being turned. When the steering wheel 101 is being turned, the duty of the control signal provided to the motor 106 changes depends on the rotational speed of the steering wheel 101. Namely, the duty of the control signal increases as the rotational speed of the steering wheel 101 becomes faster.
As described above, in the power steering system having the above described configuration, the motor 106 is controlled by using the control signal generated by the controller 104, so that the angle of the wheels 110 is controlled according to the steering operation of the steering wheel 101.
Furthermore, in the system having the above described configuration, the angle of the wheels 110 is controlled by using an oil pressure. However, the thrust required to change the angle of the wheels 110 may differ depending on the off-center angle of the wheels 110 at that timing, as shown in FIG. 2B. Namely, when the wheels 110 are in a neutral state, and when the off-center angle of the wheels 110 against the car body is small, the thrust required to change the angle of the wheels 110 is relatively small. In the meantime, when the off-center angle of the wheels 110 against the car body becomes large, the thrust required to change the angle of the wheels 110 increases. This results from a configurations of the PS cylinder 108 and a rod 109 and so on.
As described above, when the steering wheel 101 is not being turned, the revolution speed of the motor 106 is set to be low so as to consume less power. Thus, when the steering wheel 101 starts to be turned, the motor 106 is being driven at a duty of 30 percent at this timing, as shown in FIG. 2A. Therefore, at this timing, the revolution speed of the motor 106 is low, and the output generated by the oil pump 105 is small.
Accordingly, if the steering wheel 101 starts to be turned when the wheels 110 are in the neutral state or in a state close thereto, the steering operation is smooth. However, if the steering wheel 101 starts to be turned when the off-center angle of the wheels 110 against the car body is large, the steering operation feels heavy.
An object of the present invention is to provide a power steering system which consumes less power, and implements a smooth steering operation in all cases.
With the power steering system according to the present invention, the thrust (such as an oil pump output) for changing the angle of a wheel is determined based on the rotational speed of a steering wheel and the off-center angle of the wheel.
With this configuration, since the output for generating a thrust is determined based on the rotational speed of a steering wheel and the off-center angle of a wheel, even if the thrust required to change the angle of the wheel differs depending on the off-center angle of the wheel at a timing when the steering wheel is steered, a necessary and sufficient thrust can always be obtained, thereby a smooth steering operation can be implemented in all cases.
A power steering system according to another preferred embodiment of the present invention assumes a hydraulic system. This power steering system comprises a rotational speed detector detecting the rotational speed of a steering wheel; an angle detector detecting the off-center angle of a wheel; an oil pump generating the output for changing the angle of the wheel; and a controller driving the oil pump based on the rotational speed of the steering wheel, which is detected by the rotational speed detector, and the off-center angle of the wheel, which is detected by the angle detector.
Also with the system having this configuration, a smooth steering operation can be implemented in all cases by a function similar to that of the system according to the above described preferred embodiment. Specifically, for example, the controller controls the output of the oil pump so that the thrust generated in the case where the off-center angle of the wheel is large becomes larger than that generated in the case where the off-center angle of the wheel is small, when the steering wheel stops.